Photochemical electrode and method for generating photochemical electrode

ABSTRACT

A photochemical electrode includes: an electrically-conductive layer; and a photo-excited material layer including a photo-excited material provided over the electrically-conductive layer, wherein in a surface of the photo-excited material layer, a lattice plane having highest atomic density in a crystal structure of the photo-excited material is oriented in a surface direction of the surface of the photo-excited material layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-091299, filed on Apr. 28,2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a photochemicalelectrode and a method for generating a photochemical electrode.

BACKGROUND

Because of global environmental problems and energy problems, cleanenergy creation techniques without carbon dioxide (CO₂) emissions areattracting attention.

A related art is disclosed in Japanese Laid-open Patent Publication No.2015-200016.

SUMMARY

According to an aspect of the embodiment, a photochemical electrodeincludes: an electrically-conductive layer; and a photo-excited materiallayer including a photo-excited material provided over theelectrically-conductive layer, wherein in a surface of the photo-excitedmaterial layer, a lattice plane having highest atomic density in acrystal structure of the photo-excited material is oriented in a surfacedirection of the surface of the photo-excited material layer.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one example of a wurtzite crystal structure of a(1-x)GaN-xZnO (gallium nitride-zinc oxide) solid solution; and

FIG. 2 illustrates one example of a section of a photochemicalelectrode.

DESCRIPTION OF EMBODIMENTS

For example, in an artificial photosynthesis technique included in cleanenergy creation techniques, by using energy of sunlight, electronsgenerated from a photosemiconductor material (anode electrode part) arecaused to react with protons in water and hydrogen is formed. When CO₂is dissolved in the water, organic compounds serving as a source ofenergy, such as formic acid, formaldehyde, methane, and methanol, aregenerated from reaction of the electrons, the protons, and the CO₂.

In an artificial photosynthesis system, for example, electricalconduction is made by a conductor between an anode electrode and acathode electrode set in water. A photocatalytic semiconductor material(semiconductor material that may be excited by visible light and has asmall band gap) formed on the anode electrode is irradiated withsunlight and charge-hole separation is carried out, so that excitedelectrons are transmitted by the conducting line. Thus, protons and theelectrons react on the cathode electrode and hydrogen is generated.

For example, in the artificial photosynthesis system, the formedelectrons and holes may recombine when the photocatalytic semiconductormaterial formed on the anode electrode is irradiated with sunlight. Forthis reason, the amount of charge for use for the hydrogen generationreaction may decrease and a high photocurrent for the hydrogengeneration may not be obtained.

The photochemical electrode has at least an electrically-conductivelayer and a photo-excited material layer and may further include othermembers according to need.

The photo-excited material layer contains a photo-excited material. Inthe surface of the photo-excited material layer, the lattice planehaving the highest atomic density in the crystal structure of thephoto-excited material is oriented in the surface direction of thesurface of the photo-excited material layer.

As described above, in the artificial photosynthesis system, the formedelectrons and holes may recombine when the photocatalytic semiconductormaterial formed on the anode electrode is irradiated with sunlight. Forthis reason, the amount of charge for use for the hydrogen generationreaction may decrease and a high photocurrent for the hydrogengeneration may not be obtained.

For example, in the photo-excited material, charge separation due tophotoexcitation may actively occur in the lattice plane having thehighest atomic density in the crystal structure. For this reason, a highphotocurrent may be obtained by orienting the lattice plane having thehighest atomic density in the crystal structure of the photo-excitedmaterial in the surface of the photo-excited material layer.

As long as the electrically-conductive layer is a layer includingelectrical conductivity, the material, shape, size, and structure of theelectrically-conductive layer are not particularly limited and may beselected as appropriate according to the purpose. Examples of thematerial of the electrically-conductive layer include metals, metaloxides, and so forth. Examples of the metals include silver (Ag), gold(Au), copper (Cu), platinum (Pt), palladium (Pd), tungsten (W), nickel(Ni), tantalum (Ta), bismuth (Bi), lead (Pb), indium (In), tin (Sn),zinc (Zn), titanium (Ti), aluminum (Al), and so forth. Examples of themetal oxides include tin-doped indium oxide (ITO), fluorine-doped tinoxide (FTO), antimony-doped tin oxide (ATO), zinc oxide (ZnO), indiumoxide (In₂O₃), aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide(GZO), tin oxide, zinc oxide-tin oxide series, indium oxide-tin oxideseries, zinc oxide-indium oxide-magnesium oxide series, and so forth.

If the electrically-conductive layer is a thin film, theelectrically-conductive layer may be supported by a support body.Examples of the support body include a glass plate and so forth.

The photo-excited material layer contains the photo-excited material. Inthe surface of the photo-excited material layer, the lattice planehaving the highest atomic density in the crystal structure of thephoto-excited material is oriented in the surface direction of thesurface of the photo-excited material layer. The photo-excited materialmeans a material that absorbs light and is excited. The surface of thephoto-excited material layer is the surface on the opposite side to theelectrically-conductive layer side.

“The lattice plane having the highest atomic density is oriented in thesurface direction of the surface of the photo-excited material layer”means that, in the surface of the photo-excited material layer, thelattice plane having the highest atomic density in the crystal structureof the photo-excited material is exposed substantially in parallel tothe surface direction of the surface of the photo-excited materiallayer. The exposure of the lattice plane substantially in parallel tothe surface direction of the surface of the photo-excited material layerin the surface of the photo-excited material layer may be confirmed bylow-angle annular dark-field scanning transmission electron microscopy(LAADF-STEM) or the like, for example. In the surface of thephoto-excited material layer, the lattice surface having the highestatomic density in the crystal structure of the photo-excited materialmay not need to occupy the whole of the surface. If a diffraction peakby x-ray diffraction (XRD) is different from other diffraction peaks inthe relative intensity ratio compared with diffraction peaks of anisotropic material, the surface of the diffraction peak indicating thehigher intensity is oriented.

For example, when photo-excited material particles are crushed, theparticles readily break along a plane across which the bonding strengthbetween the atoms is weak (cleavage plane). The plane that becomes thecleavage plane may be the lattice plane having the highest atomicdensity in the crystal structure. When photo-excited material particlesare crushed along the cleavage plane, the particles after the crushinghave a flattened shape including the cleavage plane in the surface. Forthis reason, if photo-excited material particles fly while being crushedand are deposited on the electrically-conductive layer to form thephoto-excited material layer, in the surface of the formed photo-excitedmaterial layer, the cleavage plane, e.g. the lattice plane having thehighest atomic density in the crystal structure of the photo-excitedmaterial, is exposed substantially in parallel to the surface directionof the surface of the photo-excited material layer. Such a state may bethe state in which “the lattice plane having the highest atomic densityin the crystal structure of the photo-excited material is oriented inthe surface direction of the surface of the photo-excited materiallayer.” Also when photo-excited material particles that are not cleavedare deposited on the electrically-conductive layer and the photo-excitedmaterial layer is formed, the lattice plane having the highest atomicdensity in the crystal structure of the photo-excited material may beexposed substantially in parallel to the surface direction of thesurface of the photo-excited material layer in the surface of thephoto-excited material layer. However, the degree of exposure in thiscase may be very low and such a state may not be the state in which “thelattice plane having the highest atomic density in the crystal structureof the photo-excited material is oriented in the surface direction ofthe surface of the photo-excited material layer.”

The photo-excited material is not particularly limited and may beselected as appropriate according to the purpose. Examples of thephoto-excited material include oxides, nitrides, carbides, sulfides,III-V group compound semiconductors, II-VI group compoundsemiconductors, and so forth. One kind of material among these materialsmay be used alone or two or more kinds of materials may be used incombination.

The photo-excited material may include a wurtzite crystal structure. Ifthe photo-excited material includes a wurtzite crystal structure, thelattice plane (lattice plane having the highest atomic density in thecrystal structure of the photo-excited material) is (0001) plane.

Examples of the photo-excited material including a wurtzite crystalstructure include GaN, ZnO, BeO, and so forth. The photo-excitedmaterial including a wurtzite crystal structure may be a solid solutionof MN (M is at least any of Ga, Al, and In) and ZnO. Examples of such asolid solution include gallium nitride-zinc oxide solid solutions,aluminum nitride-zinc oxide solid solutions, indium nitride-zinc oxidesolid solutions, and so forth. Note that Ga, Al, and In share a commonground of being a group-13 element.

FIG. 1 illustrates one example of a wurtzite crystal structure of a(1-x)GaN-xZnO solid solution. In FIG. 1, the upward plane is (0001)plane and is the lattice plane having the highest atomic density in thecrystal structure of the photo-excited material.

The method for fabricating the photo-excited material layer is notparticularly limited and may be selected as appropriate according to thepurpose because it suffices that, in the surface of the photo-excitedmaterial layer, the lattice plane having the highest atomic density inthe crystal structure of the photo-excited material is oriented in thesurface direction of the surface of the photo-excited material layer.For example, the photo-excited material layer may be fabricated by anaerosol-type nanoparticle deposition (NPD) method. For example, anaerosol-type nanoparticle deposition (NPD) method described in aliterature (ADVANCED ENGINEERING MATERIALS, 2013, 15, No. 11, 1129-1135)may be used. For example, raw material particles are crushed in a nozzleas in a schematic diagram illustrated in FIG. 1(a) of the literature.When the raw material particles are crushed, the particles readily breakalong a plane across which the bonding strength between the atoms isweak, e.g. the cleavage plane. The cleavage plane is the lattice planehaving the highest atomic density in the crystal structure. For example,the photo-excited material layer may be easily fabricated when themethod described in the literature (ADVANCED ENGINEERING MATERIALS,2013, 15, No. 11, 1129-1135) is used.

The degree at which the lattice plane having the highest atomic densityin the crystal structure of the photo-excited material is oriented inthe surface direction of the surface of the photo-excited material layerin the surface of the photo-excited material layer may be referred to asthe degree of orientation. The degree of orientation may be obtained asfollows for example.

For the degree of orientation, an X-ray diffraction result is calculatedby the March-Dollase (MD) function. The MD function method is a methodin which the multiplicity of the orientation direction is multiplied bythe weight of an elliptical shape to calculate the intensity in theRietveld analysis. The degree of orientation is 0% in a random array asin powders, and the degree of orientation becomes higher as particleshaving a flattened shape increase. For example, the aspect ratio offlattened particles is calculated through peak fitting by the MDfunction. The orientation percentage (volume percentage) in the aspectratio is obtained from a curve resulting from integrating the MDfunction obtained by using the value of the aspect ratio of theflattened particles with respect to the orientation angle. For example,the analysis target may be certified to be “oriented” if the degree oforientation obtained by the above-described method is equal to or higherthan 5%.

In the photo-excited material layer, it is preferable that theoxidation-reduction potential of H⁺/H₂ and the oxidation-reductionpotential of O₂/H₂O exist between the upper edge of the valence band andthe lower edge of the conduction band in the surface on the oppositeside of the surface on the electrically-conducive layer side. Thus,oxidation decomposition of water may be carried out only with aphotochemical electrode.

The average thickness of the photo-excited material layer is notparticularly limited and may be set as appropriate according to thepurpose and may be 0.5 to 5 μm, for example.

FIG. 2 illustrates one example of a section of a photochemicalelectrode. The photochemical electrode of FIG. 2 includes anelectrically-conductive layer 1 and a photo-excited material layer 2 onthe electrically-conductive layer 1. The electrically-conductive layer 1may be supported by a support body such as a glass substrate.

The photochemical electrode is useful as an anode electrode used as ananode of carbon dioxide reduction apparatus that carries out artificialphotosynthesis. The carbon dioxide reduction apparatus includes thephotochemical electrode, for example, an anode electrode, a protonpermeable membrane, and a cathode electrode and may further includeother members according to need.

The following photo-excited materials may be used.

-   -   GaN powders: gallium nitride having an average particle size of        500 nm    -   ZnO powders: zinc oxide having an average particle size of 500        nm    -   Al₂O₃ powders: aluminum oxide having an average particle size of        500 nm

The following photo-excited material is prepared. The preparation may becarried out by a publicly-known method. In the following photo-excitedmaterial, a numeric value in parentheses represents a mole ratio. Forexample, a GaN(60)-ZnO(40) solid solution is a solid solution in whichthe mole ratio of GaN and ZnO (GaN:ZnO) is 60:40.

-   -   GaN(60)-ZnO(40) solid solution (average particle size is 500 nm)

The GaN(60)-ZnO(40) solid solution is prepared by mixing Ga₂O₃ particlesand ZnO particles in such a manner that the mole ratio of GaN and ZnO(GaN:ZnO) in the solid solution becomes 60:40 followed by performingheat treatment under a nitrogen atmosphere or an ammonia atmosphere at700° C. for 30 hours.

As a first example, glass on which an FTO (fluorine-doped tin oxide)thin film is formed is used. The GaN(60)-ZnO(40) solid solution issprayed onto the FTO thin film by an aerosol-type nanoparticledeposition (NPD) method and a thin film having an average thickness of 1μm is formed. Subsequently, annealing in a nitrogen atmosphere at 600°C. for 30 minutes is carried out. Through the above, the photochemicalelectrode is obtained. The aerosol-type nanoparticle deposition (NPD)method may be the method described in the literature (ADVANCEDENGINEERING MATERIALS, 2013, 15, No. 11, 1129-1135).

For the degree of orientation of (0001) plane, an X-ray diffractionresult may be calculated by the MD function. The MD function method is amethod in which the multiplicity of the orientation direction ismultiplied by the weight of an elliptical shape to calculate theintensity in the Rietveld analysis. The degree of orientation is 0% in arandom array as in powders, and the degree of orientation becomes higheras particles having a flattened shape increase. The calculation resultis represented in Table 1.

The obtained photochemical electrode is immersed in a Na₂SO₄ 0.5-molwater-based electrolyte, and an end part is coupled to a metal line anda platinum plate is disposed as the counter electrode. The photochemicalelectrode is disposed between both electrodes of an external powersupply and a current measurement unit (potentiostat). The surface of thephotochemical electrode is irradiated with pseudo-sunlight of 1 SUN with100 mA/cm² and the current value at this time is measured. The obtainedphotocurrent value is represented in Table 1.

In a second example, a photochemical electrode is fabricated similarlyto the first example except for that the flow rate of the carrier gas inthe first example is changed from 100 to 200 m/second. Regarding thefabricated photochemical electrode, the degree of orientation and thephotocurrent value are obtained similarly to the first example. Theresult is represented in Table 1.

In a third example, a photochemical electrode is fabricated similarly tothe first example except for that the flow rate of the carrier gas inthe first example is changed from 100 to 300 m/second. Regarding thefabricated photochemical electrode, the degree of orientation and thephotocurrent value are obtained similarly to the first example. Theresult is represented in Table 1.

In a fourth example, a photochemical electrode is fabricated similarlyto the first example except for that the flow rate of the carrier gas inthe first example is changed from 100 to 400 m/second. Regarding thefabricated photochemical electrode, the degree of orientation and thephotocurrent value are obtained similarly to the first example. Theresult is represented in Table 1.

In a fifth example, glass on which a fluorine-doped tin oxide (FTO) thinfilm is formed is used. A dispersion liquid obtained by dispersingparticles of the GaN(60)-ZnO(40) solid solution in water is applied onthe FTO thin film by using a squeegee and thereafter the water componentis removed, so that a thin film having an average thickness of 1 μm isformed. Subsequently, annealing in a nitrogen atmosphere at 600° C. for30 minutes is carried out. Through the above, a photochemical electrodeis obtained.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 1.

TABLE 1 exam- exam- exam- exam- exam- ple 1 ple 2 ple 3 ple 4 ple 5Photo-excited GaN(60)—ZnO(40) solid solution material Degree of 5 10 2035 0 orientation (%) Photocurrent 0.1 0.2 0.5 0.8 0.005 (mA/cm²)

In a sixth example, a photochemical electrode is fabricated similarly tothe second example except for that the GaN(60)-ZnO(40) solid solution inthe second example is replaced by GaN powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

In a seventh example, a photochemical electrode is fabricated similarlyto the second example except for that the GaN(60)-ZnO(40) solid solutionin the second example is replaced by ZnO powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

In a eighth example, a photochemical electrode is fabricated similarlyto the second example except for that the GaN(60)-ZnO(40) solid solutionin the second example is replaced by Al₂O₃ powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

In an ninth example, a photochemical electrode is fabricated similarlyto the first example except for that the GaN(60)-ZnO(40) solid solutionin the first example is replaced by GaN powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

In a tenth example, a photochemical electrode is fabricated similarly tothe first example except for that the GaN(60)-ZnO(40) solid solution inthe first example is replaced by ZnO powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

In a eleventh example, a photochemical electrode is fabricated similarlyto the first example except for that the GaN(60)-ZnO(40) solid solutionin the first example is replaced by Al₂O₃ powders.

Regarding the fabricated photochemical electrode, the degree oforientation and the photocurrent value are obtained similarly to thefirst example. The result is represented in Table 2.

TABLE 2 exam- exam- exam- exam- exam- exam- ple 6 ple 7 ple 8 ple 9 ple10 ple 11 Photo-excited GaN ZnO Al₂O₃ GaN ZnO Al₂O₃ material Degree of10 10 10 0 0 0 orientation (%) Photocurrent 0.05 0.1 0.05 0.002 0.0030.001 (mA/cm²)

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A photochemical electrode comprising: an electrically-conductive layer; and a photo-excited material layer including a photo-excited material provided over the electrically-conductive layer, wherein in a surface of the photo-excited material layer, a lattice plane having highest atomic density in a crystal structure of the photo-excited material is oriented in a surface direction of the surface of the photo-excited material layer.
 2. The photochemical electrode according to claim 1, wherein the photo-excited material includes a wurtzite crystal structure, and the lattice plane is (0001) plane.
 3. The photochemical electrode according to claim 2, wherein the photo-excited material including the wurtzite crystal structure is a solid solution of MN, M is at least any of gallium, aluminum, and indium, and zinc oxide.
 4. A method for generating a photochemical electrode, the method comprising: preparing an electrically-conductive layer that contains a metal or a metal oxide; spraying a photo-excited material onto the electrically-conductive layer and forming a layer; and annealing the electrically-conductive layer and the layer in a gas atmosphere.
 5. The method for generating the photochemical electrode according to claim 4, wherein the photo-excited material is a gallium nitride-zinc oxide solid solution.
 6. The method for generating the photochemical electrode according to claim 4, wherein the spraying is carried out by an aerosol-type nanoparticle deposition (NPD) method. 